The present invention relates generally to fastener-driving tools used to drive fasteners into workpieces, and specifically to combustion-powered fastener-driving tools, also referred to as combustion tools or combustion nailers. The invention is specifically directed towards lockout devices for retaining the combustion chamber of such combustion tools closed pending return of the piston to a prefiring position.
Combustion-powered tools are known in the art. Representative tools are manufactured by Illinois Tool Works, Inc. of Glenview, Ill. for use in driving fasteners into workpieces, and are described in commonly assigned patents to Nikolich U.S. Pat. Re. No. 32,452, and U.S. Pat. Nos. 4,522,162; 4,483,473; 4,483,474; 4,403,722; 5,133,329; 5,197,646; 5,263,439 and 6,145,724 all of which are incorporated by reference herein.
Such tools incorporate a tool housing enclosing a small internal combustion engine or power source. The engine is powered by a canister of pressurized fuel gas, also called a fuel cell. A battery-powered electronic power distribution unit produces a spark for ignition, and a fan located in a combustion chamber provides for both an efficient combustion within the chamber, while facilitating processes ancillary to the combustion operation of the device. Such ancillary processes include: mixing the fuel and air within the chamber; turbulence to increase the combustion process; scavenging combustion by-products with fresh air; and cooling the engine. The engine includes a reciprocating piston with an elongated, rigid driver blade disposed within a cylinder body.
A valve sleeve is axially reciprocable about the cylinder and, through a linkage, moves to close the combustion chamber when a work contact element at the end of the linkage is pressed against a workpiece. This pressing action also triggers a fuel-metering valve to introduce a specified volume of fuel into the closed combustion chamber.
Upon the pulling of a trigger switch, which causes the spark to ignite a charge of gas in the combustion chamber of the engine, the combined piston and driver blade is forced downward to impact a positioned fastener and drive it into the workpiece. The piston then returns to its original or pre-firing position, through differential gas pressures within the cylinder. Fasteners are fed magazine-style into the nosepiece, where they are held in a properly positioned orientation for receiving the impact of the driver blade. Upon ignition of the combustible fuel/air mixture, the combustion in the chamber causes the acceleration of the piston/driver blade assembly and the penetration of the fastener into the workpiece if the fastener is present.
Traditionally, combustion-powered tools have been designated as sequentially operated. In other words, the tool must be pressed against the workpiece, collapsing the workpiece contact element (WCE) before the trigger is pulled for the tool to fire or drive a nail. This contrasts with pneumatic tools, which can be fired or activated in a repetitive cycle operational format. In other words, the latter tools will fire repeatedly by pressing the tool against the workpiece, if the trigger is held in the depressed mode. These differences manifest themselves in the number of fasteners that can be fired per second for each style tool and for each mode of operation. Another aspect of sequential operation of combustion nailers is that only after a valve sleeve position switch, commonly referred to as a “chamber switch” and a trigger switch have been closed in the order mentioned and then opened, will a subsequent engine cycle be permitted. Such an operational control, described in U.S. Pat. No. 5,133,329, incorporated by reference, prevents unwanted ignition or other tool feature operations, such as electronic fuel injection (EFI), in instances when both switches remain closed after an engine cycle is complete.
One distinguishing feature that limits combustion-powered tools to sequential operation is the manner in which the drive piston is returned to the initial position after the tool is fired. Combustion-powered tools utilize self-generative vacuum to perform the piston return function. Piston return of the vacuum-type requires significantly more time than that of pneumatic tools that use positive air pressure from the supply line for piston return.
With combustion-powered tools of the type disclosed in the patents incorporated by reference above, by firing rate and control of the valve sleeve the operator controls the time interval provided for the vacuum-type piston return. The formation of the vacuum occurs following the combustion of the mixture and the exhausting of the high-pressure burnt gases. With residual high temperature gases in the tool, the surrounding lower temperature aluminum components cool and collapse the gases, thereby creating a vacuum. In many cases, such as in trim applications, the operator's cycle rate is slow enough that vacuum return works consistently and reliably.
However, for those cases where a tool is operated at a much higher cycle rate, the operator can open the combustion chamber during the piston return cycle by removing the tool from the workpiece. This causes the vacuum to be lost and piston travel will stop before reaching the top of the cylinder. This leaves the driver blade in the guide channel of the nosepiece, thereby preventing the nail strip from advancing. The net result is no nail in the firing channel and no nail fired in the next cycle.
To assure adequate closed combustion chamber dwell time in the sequentially-operated combustion tools identified above, a chamber lockout device is linked to the trigger. This mechanism holds the combustion chamber closed until the operator releases the trigger. This extends the dwell time (during which the combustion chamber is closed) by taking into account the operator's relatively slow musculature response time. In other words, the physical release of the trigger consumes enough time of the firing cycle to assure piston return. The mechanism also maintains a closed chamber in the event of a large recoil event created, for example, by firing into hard wood or on top of another nail. It is disadvantageous to maintain the chamber closed longer than the minimum time to return the piston, as cooling and purging of the tool is prevented.
Commonly-assigned U.S. Pat. No. 6,145,724 describes a cam mechanism that is operated by the driver blade to prevent premature opening of the combustion chamber prior to return of the piston/driver blade to the pre-firing position (also referred to as pre-firing). The main deficiency of this approach is that the piston requires the use of a manual reset rod to return the piston to pre-firing if the piston does not fully return due to a nail jam or perhaps a dirty/gummy cylinder wall. A piston that does not return will cause the chamber to remain closed; therefore the tool cannot be fired again.
Another type of lockout device for combustion-powered tools is disclosed in U.S. Pat. No. 6,783,045, in which a reciprocating solenoid locking device is used to restrain the valve sleeve in the sealed position to hold the combustion chamber sealed for a predetermined amount of time during which the piston should return. It has been found that the preferred embodiment of the '045 patent requires precise spatial component relationships and corresponding timing of operations to be satisfied for reliable operation between the retractable solenoid and the mating shoulders or apertures on the valve sleeve. Such precision is difficult to maintain when mass producing the tools. Furthermore, the stressful operational environment of such tools enhances the potential for combustion-induced shock forces to damage the solenoid lockout mechanism.
Thus, there is a need for an improved combustion-powered fastener-driving tool which is capable of operating in a repetitive cycle mode. There is also a need for an improved combustion-powered fastener-driving tool which can address the special needs of delaying the opening of the combustion chamber to achieve complete piston return in a repetitive cycle mode. There is also a need for a lockout device which accommodates manufacturing-induced deviations and tolerances.